Required practical 17 · Physics · Physics Paper 1, both tiers

Density

Use appropriate apparatus to measure the volume and mass of regular solids, irregular solids and liquids to find their densities.

The exam trap. 1 m³ = 1 000 000 cm³, so 1 g/cm³ = 1000 kg/m³.

Variables

Independent (you change): The object or liquid measured
Dependent (you measure): Its density (mass ÷ volume)
Control (you keep the same): The same balance, zeroed before use
Reading volumes at eye level at the bottom of the meniscus
Consistent units

Regular solid: measure its length, width and height and calculate the volume.
Why Volume = length × width × height for a cuboid.
Measure its mass on a balance and calculate density = mass ÷ volume.
Why Density is mass per unit volume.
Irregular solid: fill a eureka can to the spout, lower the object in and collect the water that overflows in a measuring cylinder.
Why The volume of water displaced equals the object's volume.
Liquid: weigh an empty measuring cylinder, add liquid, read the volume and weigh again.
Why The difference in mass is the mass of the liquid.
Read the volume at eye level, at the bottom of the meniscus.
Why This avoids parallax error.
Convert units: 1 g/cm³ = 1000 kg/m³, and 1 m³ = 1 000 000 cm³.
Why Mixing cm³ and m³ is the most common error.

Hazard	How to reduce the risk
Spilled water on the floor.	Wipe up spills at once.
Heavy objects dropped.	Lower objects gently on a string.

Object	Mass in g	Volume in cm³	Density in g/cm³
Stone	81	30	2.7
Aluminium block	54	20	2.7
Cooking oil	36.8	40	0.92

Converting cm³ to m³ by 100 or 1000 instead of 1 000 000.
Instead 1 m = 100 cm, so 1 m³ = 100³ cm³ = 1 000 000 cm³.
Using the final water level as the object's volume.
Instead Volume = final reading − starting reading.
Reading the top of the meniscus.
Instead Read the bottom of the meniscus at eye level.